It is known to provide a coil spring, also known as a helical spring, which is characterized in particular by the cross section of its wire, the coil radius of the latter and the pitch of the helix. The cross section of the wire is conventionally circular.
In the specification of U.S. Pat. No. 4,735,403, it has been proposed to provide a wire having an elongated cross section with a surface which is generally semi-circular in the outer part of the helical spring and generally semi-elliptical in the inner part of the spring, with an arc of a circle tangential between the semi-ellipse and the semi-circle.
Such cross sections as are mentioned above are not optimal as regards the reduction in their height or depth, and in practice involve maximum stresses which are slightly greater than those which occur in a spring of wire having a circular cross section. The maximum stress, as mentioned in the above mentioned United States patent specification, is at 55 degrees.